Welding electrodes may be used in electric resistance welding. Resistance welding techniques are widely used in industry, an example of one such use being the spot welding of car bodies. In that particular application, a welding gun fitted with a pair of co-operating electrodes may be moved in steps along a weld path. At each step, the electrodes are closed onto opposite sides of the workpieces to be welded and an electric current is passed between the electrodes. The electrical resistance presented at the interface between the workpieces may tend to cause local heating to occur, with the result that the workpieces fuse locally to form a weld nugget. The electrodes are then removed from the workpieces. On a production line basis, these steps are performed in rapid sequence and are repeated at each successive weld location.
In high volume manufacturing, particularly where robots may be employed to make the welds, it is advantageous to be able to continue in operation with few interruptions. One cause of interruption may be the need to replace electrodes from time to time. In general, it is desirable to be able to maintain reasonable weld quality without having to change electrodes overly frequently.
The electrodes may most typically be made of copper, or of a copper alloy, having relatively low electrical resistance and high current flow between the electrodes. As may be expected, the electrodes may tend to be come hot during use. The elevated temperature may be associated with a number of phenomena tending to shorten electrode life or to degrade welding performance, or both. First, the electrodes may tend to “pick up”, or stick, to the workpiece with the result that sparking and weld separation may occur as the electrodes are removed.
Second, the elevated temperatures may be such as to soften the copper electrodes, making them more prone to deformation under the applied contact pressure during welding. Such deformation may include plastic flow of the tip of the electrode, tending to make the tip flat, or squat, and tending to increase the contact area of the welding tip. This phenomenon may be referred to as “mushrooming”. An increase in contact area may tend to lead either to a cooler weld (due to reduced current density), and perhaps an incomplete weld, or to higher welding current, or a combination of the two.
Third, inasmuch as the welding electrodes in question may frequently be used to spot weld galvanized steel sheet in the manufacture of automobiles, at welding temperatures the zinc from the galvanized sheet may tend to migrate into the copper of the welding electrode, this tendency being greatest at the contact region of the electrode. This may tend, undesirably, to cause the formation of brass alloys at the electrode tip, and may tend to shorten tip life.
In general, it may be desirable to keep the electrode tip cool, and to discourage interaction between the material of the electrode tip and the material of the object to be welded or the coatings of the objects to be welded. It may be desirable to form a coating at the electrode tip to discourage migration of zinc, and to discourage plastic deformation. These coatings may be relatively thin—suggested as being of the order of a thousandth, or a few thousandths of an inch for a titanium carbide coating in one reference, and of the order of 5000 Angstroms for a ceramic coating in another reference. Where a coating is formed at the electrode tip, it may be that the life of the electrode may tend to be extended if the coating has relatively few openings or defects through which zinc can migrate, and, in the view of the present inventor, it may be desirable to discourage or delay the formation of cracks or gaps in the coating. Over time, a coating on an electrode tip may tend to degrade under the impact when the welding heads are closed onto the workpiece, in consequence of the heating, and in consequence of the chemical environment. The coating may be worn away, or may become cracked, and the electrode may approach the end of its useful life. In general, the present inventor is of the view that the longer some or all of a coating can remain in place on the electrode tip, and the greater the proportion of the electrode tip that may remain coated, the longer the life of the electrode may be in service.